As redox conditions change, there will be some resistance to change in a food's redox potential. This is known as poising capacity of food. This capacity is dependent on the concentration of the redox couple. Poising is greatest when the two components of a redox couple are present in equal amounts. Most fresh plant or animal foods have a low and well-poised O-R potential in their interior: the plants because of reducing substances such as ascorbic acid and reducing sugars and the animal tissues because of SH (sulphydryl) and other reducing groups. As long as the plant or animal cells respire and remain active, they tend to poise the O-R system at a low level, resisting the effect of oxygen diffusing from the outside. Therefore, a piece of fresh meat or a fresh whole fruit would have aerobic conditions only at and near the surface. The meat could support aerobic growth of slime-forming or souring bacteria at the surface at the same time as anaerobic putrefaction could be proceeding in the interior.
Processing procedures may alter this situation. For example, heating may reduce the poising power of the food by destroying or altering the reducing and oxidizing substances present and also allow more rapid diffusion of oxygen inward, either because of the destruction of poising substances or because of changes in the physical structure of the food. Processing also may remove oxidizing or reducing substances. For example, clear fruit juices loose reducing substances by their removal during extraction and filtration and therefore become more favourable to the growth of yeasts than the original juice containing the pulp.